Despite recent progress in the development of molecularly-targeted therapy and immunotherapy, lung cancer is still the leading cause of cancer death since such therapies are not applicable for more than half of lung cancer patients. Lung adenocarcinoma is the major pathological type of lung cancer. Lung adenocarcinoma has been further subdivided based on both genetic alterations (e.g., KRAS or EGFR mutations and ALK fusions) and differentiation state (including histopathologic subtypes such as acinar, lepidic, papillary, solid and mucinous). In most cases, such genetic alterations do not strictly correlate with the histopathologic subtypes of lung adenocarcinomas. However, genetic and molecular pathways that lead to the development of invasive mucinous adenocarcinoma of the lung (IMA) have been revealed by our team and others in the past several years (e.g., HNF4A pathway), suggesting that such molecular pathways can be targeted to treat IMA for which there is currently no effective therapy. Notably, the histology of IMA resembles that of mucus-producing cancers found in the gastrointestinal tract (e.g., pancreatic cancer), which hampers efforts to properly diagnose IMA from lung metastases originating from other mucus-producing organs. A biomarker to distinguish IMA from lung metastases has not been established. Our long-term goal is to identify a therapy for IMA. The objective here is to 1) determine whether HNF4A and its downstream genes that are specifically expressed in IMA but not in normal lung can be therapeutic targets for IMA and 2) comprehensively identify therapeutic targets and biomarkers using novel technologies such as single-cell mRNA-seq and 3D tumoroid (organoid) culture with a large number of IMA specimens (>200). The central hypothesis is that IMA is driven by distinct molecular pathways that can be therapeutically targeted. The rationale is based on our previous and preliminary studies indicating that 1) the HNF4A pathway is required for the growth of IMA and 2) such a pathway creates a specific tumor microenvironment that influences tumor-associated cells, which presents specific therapeutic targets and biomarkers. This hypothesis will be tested in the following specific aims: 1) determine whether HNF4A and/or its downstream genes can be therapeutic targets for IMA, 2) analyze the heterogeneity of IMA at a single-cell level, and 3) discover diagnostic biomarkers for IMA. The approach is innovative because it will focus on molecular pathways that have not been explored as therapeutic targets for IMA. The proposed research will: 1) utilize model mice for IMA (GEMM and PDX) and human specimens of IMA and lung metastases and 2) determine whether small molecular inhibitors and therapeutic antibodies can reduce the growth of IMA. The proposed research is significant because our team, comprised of a number of IMA researchers from several institutes, will bring their knowledge and expertise together to develop a novel therapy for IMA.